U.S. Pat. Nos. 4,320,220; 4,238,594, and 4,174,332; and AT-PS 366 082 disclose binders for cathodically depositable coating compositions (CED-paints) which, due to their chain end double bonds, crosslink on stoving substantially by ethylenic polymerization. Binders of this type have been used in practice on a large scale, particularly by the automobile industry, whereby thermic crosslinking has to be effected with an object, i.e., a car body, at a temperature of from about 170.degree. to 180.degree. C. in order to attain the high protection against corrosion as required by the industry. For technological reasons and to save energy, industry now demands to a rising extent the reduction of the necessary curing temperatures for the primer to a temperature of from about 150.degree. C. to a maximum of 170.degree. C. It is essential, however, that the corrosion resistance of the primer coating obtained or of the total paint coating system not be decreased.
In general, the binders disclosed in the above-mentioned references provide satisfactory crosslinking of the paint film at reduced stoving temperature. In practice, however, it has been found that with the reduced stoving temperatures of between about 140.degree. C. and a maximum of 170.degree. C., coatings subsequently applied to the CED-primer cause the primer to swell at the surface of the primer. The tendency to swell at the surface of the primers, which increases with decreasing stoving temperature, is believed to be caused by the strong solvency power of the organic solvents contained in the coatings applied to the primer. This effect shows strongest when the coating is applied immediately after baking of the primer, and diminishes on storage of the primer coated objects before application of an additional coating. For example, a primer cured at 150.degree. C. is totally resistant to such solvents after a maximum storage of three weeks. These changes in the properties at the surface of the primer upon storage, in many cases, are the reason for the improvement in the multiple-coat system through hardening of the subsequent filler and finishing coatings which results in an increased impact resistance of the final paint system. Intercoat adhesion is also essentially enhanced for materials used as protective materials for car underbodies or to sealants used on car bodies with aging of the primer. It is evident, however, that such long storage times cannot be realized in practice. Normally, the following coats are applied to the primer within a maximum period of one hour after stoving.